Many hydrometallurgical processes generate large quantities of by product ferrous containing liquors. Examples of such processes include the recovery of uranium, copper, nickel, cobalt, zinc from their ores, ore concentrates or tailings. The ferrous ions represent a potential, inexpensive source of oxidant, which can be oxidised to ferric ions and recycled for use as an oxidant to one or more stages of the process.
One industry approach of oxidising ferrous ions is by treating the ferrous solution with oxygen and sulfur dioxide gas which are available at some sulfide ore processing plants.
The ferrous oxidation reaction can be written as follows:2FeSO4+O2(aq)+SO2(aq)→Fe2(SO4)3  (1)
However, if the amount of SO2 is too high relative to O2 the ferrous oxidation reaction is undesirably reversed and ferric ions are reduced to ferrous ions. The ferric reduction reaction can be written as:Fe2(SO4)3+SO2+2H2O→2FeSO4+H2SO4  (2)
It has previously been attempted to find a critical ratio between gaseous oxygen and sulfur dioxide, above which the ferrous to ferric reaction occurs and below which ferric is reduced to ferrous, and to operate the ferrous oxidation process above that critical ratio.
The inventor has recognised that this approach is problematical because the reactions take place in the aqueous phase and the rate of diffusion between the introduced gases and the liquor is dependent on a number of factors, such as:                the partial pressure of oxygen in the gas;        the configuration of the reactor in which the oxidation process takes place, in particular its height (that is the hydrostatic pressure within the reactor);        the degree of agitation of the ferrous solution within the reactor, in particular the type of agitator used and its power; and        the rate of utilisation of the dissolved oxygen in solution by chemical reaction.        
The inventor has also recognised that while it may be possible to determine an optimal gaseous sulfur dioxide to oxygen ratio for particular process conditions and/or a particular reactor, that ratio is unlikely to be applicable under different process and/or reactor conditions, for example, where an operation is being scaled up from the laboratory or pilot plant to commercial scale.
It is accordingly an object of the present invention to provide a process for the controlled oxidation of a ferrous ion containing solution which overcomes or at least alleviates one or more of the above discussed disadvantages of the prior art.